Conventional linear regulator is accomplished by the linear regulation of the pass element (e.g., MOSFET). The pass element is used as one half of a potential divider to control the output voltage. A feedback circuit compares the output voltage to a reference voltage in order to adjust the input to the pass element, thus keeping the output voltage reasonably constant. The pass element operates in its “linear region” and acts like a variable resistor. The pass element continuously adjusts a voltage divider network to maintain a constant output voltage.
However, the linear regulation of the pass element faces stability issues due to wide load requirements. The pass element used in the conventional method is scaled based on load requirements, which will constitute an internal pole. Therefore, the output pole which shifts with the load current would compromise the stability issue as it moves too close to the internal pole of the conventional linear regulator. In other words, the regulator stability becomes an issue because of the interaction of this internal pole with the pole at the regulator's output.
There are a lot of existing methods for voltage regulators. Virtually, all of these existing methods deal with the tough constraints imposed by the use of the pass element, which usually gives an internal pole.
In that sense, any regulator with the pass element having amplification (e.g. a common-source connected PMOS) will have issues with the loop stability. Thus, stability is among the most difficult issues to address in traditional linear regulators.